1. Field of the Invention
The present invention relates to overhead projectors and, in particular, illumination systems for use in such projectors.
2. Description of the Prior Art
Typical overhead projectors comprise a box-like case containing an illumination system beneath a Fresnel condensing lens, an adjacent stage for supporting a transparency, and a projection head adjustably mounted above the stage for directing and focusing the projected image. The illumination system includes optics and a light source, usually a tungsten filament lamp. The optics collect as much light as possible from the light source and distribute this light over the Fresnel lens.
In order to utilize the light emitted from the backside of the light source filament a concave reflector is often placed behind the lamp filament. In the most common configuration, the reflector is spherical and is positioned such that the lamp filament is at the reflector's center of curvature, thus producing an inverted image of the lamp filament at unit magnification and at the same focal plane as the lamp filament. When the tungsten filament is constructed in an open or loosely wound fashion, the position of the reflected image is usually adjusted laterally to fill in gaps in the filament structure, and thus prevent obstruction of the reflected filament image by the actual filament. A description of this construction is found in Engineering Optics, Habel and Cox, Pitman Publishing, 1971, pp. 251-252. If the tungsten filament is of a closed or tightly wound construction, the filament image is laterally shifted to one side of the actual filament, again to prevent obstruction of the reflected filament image by the actual filament. A discussion of this construction may be found in Optics and Optical Instruments, Johnson, Dover Publications, Inc., 1960, page 141. In both cases, screen illumination is increased by use of the concave reflector.
U.S. Pat. No. 3,979,160, assigned to the assignee of the present invention, describes an improved illumination system which achieves improved lighting efficiency and uniformity by the use of an ellipsoidal reflector and a de-focused light source. The filament image is longitudinally separated from the actual filament to such a degree that the filament image becomes the only effective source of light. It is claimed that with this illumination system 2,000 screen lumens can be achieved with a 360 watt lamp.